1. Field of the Invention
The present invention relates generally to the treatment of effluent gas streams to remove contaminant vapors therefrom. More particularly, the invention relates to the removal of such vapors by a plasma-enhanced extraction process.
Semiconductor fabrication processes use large amounts of a variety of toxic, corrosive, and flammable gases. For example, chemical vapor deposition (CVD) processes utilize large volumes of silane, dichlorosilane, ammonia, nitric oxides, arsine, phosphine, diborine, boron trichloride, and the like, only a small portion of which are actually consumed in the process. Thus, the waste streams discharged from these processes contain relatively high concentrations of very toxic species. It has become an environmental and legal imperative to remove such toxic species prior to release of the effluent gas streams to the atmosphere.
A variety of conventional gas treatment methodologies have been adapted to treat the gas effluents from semiconductor fabrication processes. One of the most common approaches is to decompose, react, or combust the contaminants at high temperatures. For example, silane may be burned with oxygen or air, producing silicon dioxide particles. Unfortunately, silane combustion suffers from a number of drawbacks. First, the silicon dioxide particles form a very fine powder (as a result of gas phase reaction) which can clog the burner and frequently leads to failure of the combustion system. Second, the particles are generally collected by water scrubbing, and the scrubbing water must itself be treated prior to disposal to remove the particles as well as various water soluble contaminants.
Scrubbing with water, chemical solutions, and dry chemicals have also been used to treat waste effluent gases from semiconductor fabrication processes. Water scrubbing may be used to dissolve water soluble components of the effluent gas. For contaminants which are insoluble or sparingly soluble in water, chemical scrubbing can be used. Although effective, the water or chemical absorbent must be treated prior to release from the plant. With increasingly stringent controls on water pollution, such scrubbing techniques are becoming less attractive. Dry chemical adsorption and/or reaction may also be relied on to extract the contaminant species from the gas. Unfortunately, such dry chemical scrubbing is inefficient for treating large volumes of gaseous effluents.
The most widely used treatment method is dilution. That is, the concentrations of toxic species are reduced by combining the waste effluent gas stream with large volumes of air or an inert diluent gas. While such reduction of concentration levels may literally satisfy existing effluent limits, they in fact result in no actual reduction in the amount of toxic species released to the atmosphere. Moreover, as pollution regulations are tightened in the future, the use of dilution as a treatment methodology will become less acceptable.
Plasma-enhanced systems have also been proposed for treatment of effluent gases. Such systems, however, have heretofore relied on gas phase reaction which usually results in the formation of a very fine powder which itself presents substantial collection and disposal problems.
For the above reasons, it would be desirable to provide methods and apparatuses which are capable of substantially completely removing vapor phase contaminants from gaseous waste effluent streams. It would be particularly desirable if such methods resulted in the conversion of the vapor phase contaminants to a solid phase which is amenable to containment. Ideally, the method will result in the deposition of a solid phase layer of an inert material on a disposable substrate (rather than a fine powder or dust), so that the contaminants are prevented from entering the air and/or water. Such methods should also be economic, requiring only moderate capital and operating expenses, and should be easily incorporated into existing semiconductor fabrication plants so that they are likely to be utilized, and should be capable of handling relatively large volumes of gaseous effluents. 2. Description of the Background Art
Plasma-enhanced destruction of contaminants from plasma-type reactors is known. Japanese patent application No. 58-6231 teaches the use of a parallel plate plasma reactor in series with a primary CVD reactor, where the plasma reactor is used to decompose silane exhausted from the CVD reactor. German patent application No. 230,790 similarly teaches the use of a plasma to react halogen vapor effluent from a plasma etcher. Neither the Japanese nor the German application teach the deposition of the contaminants onto a solid phase for disposal. Apparently, particulates are formed in the gas phase and continually removed as a powder from the system.
Plasma-enhanced treatment of effluent gases is taught in a number of the other patents and applications, but none teach a deposition/extraction mechanism. Japanese patent application No. 51-129868, German application No. 215,706, European application No. 176,295, and Japanese application No. 52-078176 all teach the plasmaphase reaction of an effluent with a reactant to produce inert species. Japanese patent application No. 49-122879 teaches the treatment of nitrogen-oxides in a plasma by reacting with iron, aluminum, or hydrogen to produce stable compounds. Other patents teach the use of glow discharge and/or other electronic excitation to assist in decomposing waste gases. See, e.g., Japanese patent application No. 51-094178, British patent No. 2158055 (which teaches the production of solid products), European patent No. 158,823, Japanese patent No. 60-193522, Japanese patent No. 60-137421, and Japanese patent application No. 56-108514.
Japanese patent application No. 58-45718 discloses the use of spiral gas mixing eleotrodes in a gas effluent treatment system. The system, however, does not operate by deposition of the contaminants, but rather by ionization and oxidation. U.S. Pat. No. 4,491,606 discloses a particular device for spacing parallel electrodes in a conventional plasma-enhanced CVD system.